The present invention relates to the technology of thermal printing, and more particularly concerns a driver circuit for reducing print-density variations from temperature effects in the printing elements.
Thermal printers produce visible marks on specially treated paper by heating localized areas, commonly in a "dot matrix" pattern, above a threshold temperature. Although the individual print elements are small, they are usually supported on a substrate which has a considerable thermal inertia. Print-element temperature variations than produce noticeably different darknesses or densities at different times.
Previous approaches to this problem involve direct temperature measurements for adjusting the amount of energy to be applied to thermal print elements. U.S. Pat. Nos. 3,577,137 and 3,725,898, for example, create signals related to head temperatures; these systems, while compensating for variations from many different causes, are complex and expensive. U.S. Pat. No. 3,975,707 compensates for ambient air temperature, and is also complicated and expensive. Such circuits are inappropriate in a technology whose major advantage is its otherwise simplicity and low cost.